What is the equivalent kanji for this word? I cannot

What is the equivalent kanji for this word? I cannot understand it’s meaning on this sentence.

学習する(学習しない、学習した、学習する時、などと変化)
理解する(同上)

「する」は、他の語と複合して動詞をつくる。

@toni - There are certainly stand-alone words pronounced as し(shi), such as four (四), poem (詩) and city (市)etc. But Hiragana is also used for grammatical verb endings, some adjective endings, and for grammatical particles/markers, none of which are written in Kanji. In other words, you’ll see verbs and some adjectives written in a combination of kanji & hiragana.

As Yutaka shows, the main part of verbs are written in kanji, but the changing (inflecting/conjugating) ENDS of verbs are written in hiragana.

In the lesson, the verb ‘to learn’ 学習します (がくしゅうします, gakushūSHImasu), is therefore not written entirely in kanji, but as a combination of kanji and hiragana. This is why しdoesn’t have its own separate kanji here.

I think you are most likely confused by the fact that the LingQ word splitter appears to put a space between しandます, for example, which is really one word: します. I notice too that 学習したい ‘want to learn’ appears to have spaces where it appears in the lesson, but it’s really one word.

However, you’ll soon get to know these forms so well, that you won’t even notice the spaces. Real Japanese doesn’t even uses spaces, but it’s helpful for us beginners. The word splitter isn’t perfect, but you can click on ‘ignore’ or ‘known’ for the parts that don’t split correctly, so they don’t keep coming up as ‘blue words’. Then you can highlight the whole of ‘します’ and turn it into a lingQ to add to your database. I do the same thing for other words which don’t always split perfectly.

The rest might be helpful re adjectives and particles:
In the lesson you see the adjective 新しい, which is one word あたらしい, meaning ‘new’. Only the ‘a ta ra’ part is written in kanji, and the SHĪ [shii] part in hiragana. The hiragana ending grammatically changes/inflects in some types of adjectives. ‘WAS new’ will be 新しかった [a ta ra SHI KATTA]. (Just to give an idea of what I mean by changes).

Grammatical particles/markers are also written in hiragana. They indicate for example, the grammatical function of nouns, such as whether something is the topic はha (but pronounced as ‘WA’), or object を wo (but pronounced as ‘O’). Don’t worry, you’ll soon get the hang of pronouncing particles! Some words also have anお in front of them, which is an honorific prefix, egお名前は? “[What is] your name?” (O namae wa?), a more formal way of asking someone’s name. (Note the WA pronunciation for topic marker は ha) .

Good luck with your language learning!

Awesome explanation. Thank you.